Process for the synthesis of a copolyester adhesive resin

ABSTRACT

Isophthalic acid is commonly utilized as a comonomer in the preparation of copolyester resins which are utilized as solution adhesives. Unfortunately, solutions of such resins frequently become hazy due to the presence of cyclic oligomers. This invention is based upon the unexpected discovery that the presence of protonic acids during the synthesis of the copolyester resins containing isophthalic acid utilizing conventional metallic catalysts results in solutions of the resins in organic solvents being virtually haze free. The subject invention more specifically reveals a process for synthesizing a polyethylene terephthalate/isophthalate/azelate copolyester resin which is particularly useful as a solution adhesive resin, comprising reacting ethylene glycol with a diacid component which is comprised of (a) from about 25 to about 45 mole percent terephthalic acid, (b) from about 10 to about 30 to about 55 mole percent azelaic acid based upon the diacid component; in the presence of at least one metallic catalyst selected from the group consisting of antimony compounds, titanium compounds; manganese compounds, and germanium compounds and in the presence of at least one protonic acid having a pKa of less than about 2.5; wherein the amount of metallic catalyst present is within the rang eof about 130 ppm to about 1500 ppm and wherein the amount of protonic acid present is within the range of about 50 ppm to about 500 ppm, with said amounts being based upon the weight of the copolyester resin produced.

BACKGROUND OF THE INVENTION

Solution or lacquer adhesives are utilized in a wide variety ofapplications. Such adhesives are particularly useful because they can beeasily applied to a substrate.

Solution adhesives are bonding compositions which achieve a solid stateand resultant strength through evaporation or removal of solvent. Thisis in contrast to hot melt adhesives which achieve a solid state andresultant strength by cooling from an elevated temperature. In otherwords, the application of heat melts the hot melt adhesive and after theheat source is removed, it solidifies simply by cooling. Hot meltadhesives are widely used in industrial applications because they can beused to form strong bonds very quickly. However, the difficultyassociated with applying hot melt adhesives at an elevated temperaturehas greatly limited their usage in many applications.

Solution adhesives offer an advantage over hot melt adhesives in thatheating is not required in their application to a substrate. Solutionadhesives are widely utilized in general purpose and householdapplications because of their ease of application. Solution adhesivesare also the adhesive of choice in a wide variety of industrialapplications, such as those where the application of a hot melt adhesivewould be difficult or impractical.

Copolyester resins are frequently utilized in solution adhesivecompositions. Such solution adhesive compositions are simply prepared bydissolving the copolyester resin in a suitable organic solvent. It ishighly desirable for the organic solvent to have a low boiling point.This results in faster rates of solvent evaporation and consequentlyreduces the time required for bonding. The copolyester resins utilizedin solution adhesives commonly contain repeat units which are derivedfrom isophthalic acid. For example, polyethyleneterephthalate/isophthalate/azelate copolyesters are widely utilized assolution adhesive resins. Unfortunately, solutions of such copolyesterresins form visible haze on standing. Haze formation in such solutionsis, of course, highly undesirable because it renders the solutionadhesive composition unsuitable for use in many applications.

The haze which forms in such solution adhesives is due to the presenceof cyclic oligomers, such as the cyclic dimer of ethylene azelate, thecyclic dimer of ethylene isophthalate, the cyclic monomer of ethyleneazelate, and the cyclic trimer of ethylene terephthalate. The cyclicdimer of ethylene isophthalate is believed to be the predominant cyclicoligomer in such solutions. It is the result of the cyclization ofethylene glycol and isophthalic acid to form the following ringstructure: ##STR1##

This ring structure contains two units of ethylene isophthalate (hence,the name cyclic dimer) and is a by-product of the polyesterificationreaction of ethylene glycol with isophthalic acid or of thetransesterification reaction of ethylene glycol with isophthalate esterssuch as dimethyl- or diethylisophthalate.

Cyclic dimer is produced as an unwanted reaction by-product inconventional synthesis techniques for preparing copolyester resins whichcontain repeat units which are derived from isophthalic acid. In suchconventional synthesis techniques, the cyclic dimer is believed to beformed during the esterification stage of the reaction. This is becausecyclic dimer is the esterification product of two isophthalic acidmolecules with two ethylene glycol molecules.

SUMMARY OF THE INVENTION

This invention overcomes the problem of haze formation in solutionadhesive formulations which contain copolyester resins having repeatunits which are derived from isophthalic acid. It is based upon theunexpected finding that the presence of protonic acids during thesynthesis of isophthalic acid containing copolyesters results insolutions of the copolyesters in organic solvents being essentially hazefree. Accordingly, the process of this invention can be utilized in thesynthesis of isophthalic acid containing copolyesters which do not formhaze in solutions with organic solvents. The presence of protonic acidsduring the synthesis of such copolyesters may inhibit the formation ofcyclic oligomers or may improve the solubility of such cyclic oligomersin organic solvents or a combination of both of these.

The subject invention more specifically reveals in a process for thesynthesis of polyester resins having repeat units which are derived fromisophthalic acid by reacting at least one dicarboxylic acid or diesterthereof with at least one diol in the presence of at least one metalliccatalyst, wherein one of the dicarboxylic acids is isophthalic acid: theimprovement which comprises conducting the reaction in the presence of50 ppm to 500 ppm of at least one protonic acid having a pKa of lessthan about 2.5, wherein the amounts of protonic acid present are basedupon the weight of the polyester resin produced.

The present invention also discloses a process for synthesizing apolyethylene terephthalate/isophthalate/azelate copolyester resin whichis particularly useful as a solution adhesive resin, comprising reactingethylene glycol with a diacid component which is comprised of (a) fromabout 25 to about 45 mole percent terephthalic acid, (b) from about 10to about 30 mole percent isophthalic acid, and (c) from about 30 toabout 55 mole percent azelaic acid based upon the diacid component; inthe presence of at least one metallic catalyst selected from the groupconsisting of antimony compounds, titanium compounds; manganesecompounds, and germanium compounds and in the presence of at least oneprotonic acid having a pKa of less than about 2.5; wherein the amount ofmetallic catalyst present is within the range of about 130 ppm to about1500 ppm and wherein the amount of protonic acid present is within therange of about 50 ppm to about 500 ppm, with said amounts being basedupon the weight of the copolyester resin produced.

DETAILED DESCRIPTION OF THE INVENTION

The process of this invention can be utilized in the preparation ofpolyester resins which contain repeat units which are derived fromisophthalic acid or a diester thereof, such as dimethyl isophthalate ordiethyl isophthalate. Such polyesters will typically contain at leastabout 2 mole percent repeat units which are derived from isophthalicacid or a diester thereof, based upon the total diacid component in thepolyester. Such polyesters will more typically contain at least about 5mole percent repeat units which are derived from isophthalic acid or adiester thereof.

The copolyesters of this invention are prepared by reacting a diacidcomponent with a diol component. Isophthalic acid or a diester thereofis, of course, one of the diacids in the diacid component. The term"diacid component" as used herein is therefore intended to includediesters of diacids. A wide variety of diacids can be utilized inconjunction with isophthalic acid in the diacid component. For example,both aromatic and aliphatic dicarboxylic acids can be utilized in thediacid component with isophthalic acid. Phthalic acid, terephthalicacid, 2,6-napthalene dicarboxylic acid, and 1,6-napthalene dicarboxylicacid are some representative examples of aromatic dicarboxylic acidswhich can be utilized in the diacid component with isophthalic acid. Thealiphatic dicarboxylic acids which can be used in the diacid componentwill normally contain from about 4 to about 12 carbon atoms. Somerepresentative examples of suitable aliphatic dicarboxylic acids include1,4-butane dicarboxylic acid, 1,5-pentane dicarboxylic acid, 1,6-hexanedicarboxylic acid, 1,7-heptane dicarboxylic acid (azelaic acid),1,8-octane dicarboxylic acid, 1,9-nonane dicarboxylic acid, 1,10-decanedicarboxylic acid, 1,11-undecane dicarboxylic acid, and 1,12-dodecanedicarboxylic acid. The diacid component utilized in preparingcopolyester resins for solution adhesives will typically contain fromabout 25 mole percent to about 45 mole percent terephthalic acid, fromabout 10 mole percent to about 30 mole percent isophthalic acid, andfrom about 30 mole percent to about 55 mole percent azelaic acid. Suchresins for solution adhesives will preferably contain from about 30 molepercent to about 40 mole percent terephthalic acid, from about 15 molepercent to about 25 mole percent isophthalic acid, and from about 40mole percent to about 52 mole percent azelaic acid.

A wide variety of diols can be utilized in the diol component of thepolyesters of this invention. The diol utilized in the polyesters ofthis invention will typically be aliphatic diols containing from 2 toabout 8 carbon atoms. Some representative examples of diols which can beutilized in the diol component include ethylene glycol, 1,3-propanediol, 1,4-butane diol, and neopentyl glycol. Ethylene glycol is highlypreferred for utilization in preparing copolyester resins for solutionadhesives due to its generally lower cost.

The copolyesters of this invention are synthesized in the presence of atleast one protonic acid as a cyclic dimer inhibitor. However, in allother respects, the polyesters of this invention are synthesizedutilizing conventional polymerization techniques which are well known tothose skilled in the art. In other words, the polyesters are synthesizedin the presence of a protonic acid utilizing polymerization techniqueswhich are in other respects conventional. Thus, conventionaltemperatures, catalysts, amounts of catalysts, stabilizers and the like,are used in manners well known in the literature and art.

These polyesters will normally be prepared utilizing a two step processwhich consists of an esterification stage and a condensation stage. Inthe first step of the process, the esterification stage, the diacidcomponent is esterified with the diol component. This step is typicallycarried out at a temperature which is within the range of about 150° C.to about 240° C. under an inert atmosphere which is maintained atatmospheric pressure. The inert gas utilized in the esterification stageis generally nitrogen or a noble gas with nitrogen being preferred foreconomic reasons. The esterification stage of the reaction can becarried out in the presence of appropriate catalysts, such as titaniumalkoxides, tin alkoxides, tetraalkyl titanium compounds, manganesetrifluoroacetic acid, manganese acetate, lead acetate, or zinc acetates.In cases where the diacid component is a diester, a catalyst willnormally be utilized in order to attain satisfactory reaction rates.During the esterification stage of the reaction, both esterification andpolycondensation reactions occur simultaneously. However, during theesterification stage, the esterification reaction will be predominantwith condensation reactions occurring to a lesser extent.

The second step in the preparation of such polyesters is thecondensation step. During this stage of the reaction, polycondensationreactions occur with the molecular weight and intrinsic viscosity of thepolyester continuously increasing. The condensation stage of thereaction is carried out under a reduced pressure and at an increasedtemperature to attain faster rates of polymerization. For instance, thepressure utilized in the condensation stage is typically less than about25 mm of mercury (2.7×10³ Pascals) and preferably less than about 5 mmof mercury (6.7×10² Pascals) It is most preferred to utilize a reducedpressure of less than about 0.5 mm of mercury (66.7 Pascals). It isimportant to attain high vacuums when resins having high intrinsicviscosities are being synthesized. The temperature utilized in theesterification stage is typically within the range of about 230° C. toabout 300° C. The polymerization time required will vary with the amountand type of catalyst used as well as the polymerization temperature andvacuum utilized. The extent of the polycondensation will also dependsomewhat on the desired molecular weight or intrinsic viscosity of thepolyester resin being synthesized.

The copolyesters of this invention which are utilized as solutionadhesive resins will generally have relatively high intrinsicviscosities. For instance, such polyesters will normally have intrinsicviscosities of at least about 0.5 dl/g. These copolyester resins willmore preferably have intrinsic viscosities which are within the range ofabout 0.6 dl/g to about 1.0 dl/g. It is most preferred for polyesterresins which are utilized as solution adhesives to have intrinsicviscosities which are within the range of about 0.75 to about 0.95 dl/g.The intrinsic viscosity of such polyester resins are typically measuredin a 60:40 phenol:tetrachloroethane solvent system at a temperature of30° C. and at a concentration of 0.4 g/dl.

The process of this invention utilizes a protonic acid in the synthesisof copolyesters which can be dissolved in organic solvents to makeessentially haze free solutions. The protonic acids which can beutilized typically have a pKa of less than about 2.5 and preferably lessthan about 2.0. Some representative examples of suitable protonic acidsinclude sulfuric acid, phosphoric acid, sulphurous acid, trichloroaceticacid, dichloroacetic acid, para-toluene sulfonic acid, meta-toluenesulfonic acid and the like. Organic acids, such as sulfonic acids, aregenerally preferred. Para-toluene sulfonic acid is a highly preferredprotonic acid. It is highly preferred because its utilization leads toless processing difficulties during the synthesis of the copolyester.

The protonic acid will typically be utilized in the smallestconcentration which will inhibit the formation of cyclic dimer. This istypically within the range of about 50 ppm to about 500 ppm. It isgenerally preferred to utilize a concentration of protonic acid which iswithin the range of about 75 ppm to about 200 ppm. It is typically mostpreferred to utilize a concentration of protonic acid which is withinthe range of about 100 ppm to about 150 ppm. These concentrations whichare expressed in ppm (parts per million) are calculated based upon theweight of the polyester resin produced. The polymerizations of thisinvention will typically be conducted in the presence of about 130 ppmto about 1500 ppm of at least one metallic catalyst. It is generallypreferred for the amount of metallic catalyst present to be within therange of about 160 ppm to about 1000 ppm with an amount which is withinthe range of about 200 ppm to about 400 ppm being most preferred.

Solution adhesives of the copolyesters of the present invention can beprepared by simply dissolving the copolyester in an appropriate organicsolvent. Such solvents will have a relatively low boiling point and willbe capable of dissolving a relatively large amount of the polyesterresin per unit volume. Carbon tetrachloride, chloroform,tetrachloroethane, methyl ethyl ketone and tetrahydrofuran have suitableproperties for utilization as solvents in such adhesive compositions.Methyl ethyl ketone and tetrahydrofuran are particularly preferred forutilization as solvents in such solution adhesive compositions. Suchsolution adhesives can contain up to about 40 weight percent copolyesterresin, based upon the total weight of the solution. It is generallypreferred for such solution adhesive compositions to contain from about15 weight percent to about 35 weight percent copolyester resin, basedupon the total weight of the solution adhesive composition. Suchsolutions of the polyester resins of this invention are essentially hazefree.

Antidegradants, colorants, fillers and/or other desired agents can beadded to the solution adhesives of this invention. The solution adhesivecan then be utilized in a conventional manner such as by applying it toa substrate and transferring the substrate to a surface to which it isto be adhered. Upon evaporation of the solvent, bonding of the twosurfaces results. Such solution adhesives are useful for adhering metal,plastics, natural and synthetic fabrics, paper, leather, cardboard andglass.

In order to more fully illustrate the nature of this invention and themanner of practicing the same, the following examples are presented.Unless specifically indicated otherwise, all parts and percentages aregiven by weight.

EXAMPLE 1

In this experiment a copolyester resin which could be utilized inpreparing haze-free solution adhesives was prepared in a 15 lbs. (6.8kg) reactor. In the procedure utilized, the reactor oil temperature wasset at 220° C. for the initial monomer charge. In the monomer charge,there was a molar ratio of ethylene glycol to diacids of 1.2:1. Theinitial charge contained 4.4 lbs. (2 kg) of ethylene glycol, 1519.6grams of terephthalic acid, 893.8 grams of isophthalic acid, 2326.9grams of azelaic acid, and 1.09 grams of a premixed solution containingpara-toluene sulfonic acid and antimony trioxide. The diacid componentutilized in this polymerization contained 34 mole percent terephthalicacid, 20 mole percent isophthalic acid, and 46 mole percent azelaicacid. The initial charge also contained 200 ppm of para-toluene sulfonicacid and 1,000 ppm of antimony trioxide.

After about 30 minutes, the reactor oil temperature was increased to260° C. A column head temperature of about 145° C. and a pressure ofabout 35 psig (3.4×10⁵ Pascals) was maintained. When the column headtemperature reduced to about 110° C. due to reduced generation of wateras an esterification reaction by-product, the contents of the reactorwere transferred to the second stage. The theoretical yield of water asa reaction by-product was 869 ml.

The initial reactor oil temperature for the second stage reactor was235° C. A vacuum was applied with a full vacuum of less than 1 mm of Hg(133 Pascals) ultimately being attained. The reactor oil temperature wasalso increased to 245° C. This condensation stage of the reaction wascontinued until an intrinsic viscosity for the copolyester resin beingproduced of about 0.85 d/g was attained.

The copolyester prepared was dissolved in methyl ethyl ketone. Thesolution formed remained haze free for a period of over 2 weeks. Itshould be noted that even though some copolyester solutions containingcyclic dimer are initially haze free, that haze typically forms after afew days. However, this technique was successful in producing acopolyester resin which remained haze free in solution for long periodsof time. The solution prepared was also determined to have an excellentcombination of properties for utilization as an adhesive. Such solutionadhesives are particularly useful in food packaging applications, suchas multilayer laminates for food packaging and retort pouches. They arealso useful for affixing solar films to glass.

EXAMPLE 2 (COMPARATIVE)

This experiment was conducted utilizing the same procedure described inExample 1 except that para-toluene sulfonic acid was not included in thepolymerization recipe. In this experiment, the copolyester resinproduced was also dissolved in methyl ethyl ketone to make a solutionadhesive. However, haze formed in the solution after a few days. Thus,this comparative experiment clearly shows that the presence ofpara-toluene sulfonic acid resulted in the formation of a resin whichcould be utilized in making a haze free solution in methyl ethyl ketone.

EXAMPLE 3

This experiment was conducted utilizing the same procedure as isdescribed in Example 1 except that only 400 ppm of antimony trioxide wasutilized in the polymerization. Solutions of the copolyester made inmethyl ethyl ketone had good clarity and remained haze free for over twoweeks. The color of the resin made in this experiment was deemed to bebetter than the color of the resin produced in Example 1. The resin madealso had an excellent combination of properties for utilization inmaking solution adhesives.

While certain representative embodiments and details have been shown forthe purpose of illustrating this invention, it will be apparent to thoseskilled in this art that various changes and modifications can be madetherein without departing from the scope of the present invention.

What is claimed is:
 1. In a process for the synthesis of polyesterresins having repeat units which are derived from isophthalic acid byreacting at least one dicarboxylic acid or diester thereof with at leastone diol in the presence of at least one metallic catalyst, wherein oneof the dicarboxylic acids is isophthalic acid; the improvement whichcomprises conducting the reaction in the presence of 50 ppm to 500 ppmof at least one protonic acid having a pKa of less than about 2.5,wherein the amounts of protonic acid present are based upon the weightof the polyester resin produced.
 2. A process for synthesizing apolyethylene terephthalate/isophthalate/azelate copolyester resin whichis particularly useful as a solution adhesive resin, comprising reactingethylene glycol with a diacid component which is comprised of (a) fromabout 25 to about 45 mole percent terephthalic acid, (b) from about 10to about 30 mole percent isophthalic acid, and (c) from about 30 toabout 55 mole percent azelaic acid based upon the diacid component: inthe presence of at least one metallic catalyst selected from the groupconsisting of antimony compounds, titanium compounds: manganesecompounds, and germanium compounds and in the presence of at least oneprotonic acid having a pKa of less than about 2.5; wherein the amount ofmetallic catalyst present is within the range of about 130 ppm to about1500 ppm and wherein the amount of protonic acid present is within therange of about 50 ppm to about 500 ppm, with said amounts being basedupon the weight of the copolyester resin produced.
 3. A process asspecified in claim 1 wherein the pKa of the protonic acid is less thanabout 2.0.
 4. A process as specified in claim 1 wherein the protonicacid is para-toluene sulfonic acid.
 5. A process as specified in claim 1wherein from about 75 ppm to about 200 ppm of the protonic acid catalystis present.
 6. A process as specified in claim 1 wherein from about 100to about 150 ppm of the protonic acid catalyst is present.
 7. A processas specified in claim 5 wherein the protonic acid is para-toluenesulfonic acid.
 8. A process as specified in claim 6 wherein the protonicacid is para-toluene sulfonic acid.
 9. A process as specified in claim 8wherein the metallic catalyst is an antimony compound.
 10. A process asspecified in claim 9 wherein the antimony compound is antimony trioxide.11. A process as specified in claim 2 wherein the protonic acid has apKa of less than about 2.0.
 12. A process as specified in claim 2wherein the protonic acid is present at a level which is within therange of about 75 ppm to about 200 ppm.
 13. A process as specified inclaim 12 wherein the diacid component is comprised of from about 30 toabout mole percent terephthalic acid, from about 15 to about 25 molepercent isophthalic acid, and from about to about 52 mole percentazelaic acid.
 14. A process as specified in claim 13 wherein themetallic catalyst is an antimony compound.
 15. A process as specified inclaim 14 wherein the protonic acid is para-toluene sulfonic acid.
 16. Aprocess as specified in claim 15 wherein the amount of protonic acidpresent is within the range of about 100 ppm to about 150 ppm.
 17. Aprocess as specified in claim 16 wherein the antimony compound isantimony trioxide.
 18. A process as specified in claim 17 wherein theamount of antimony trioxide present is about 1,000 ppm.
 19. A process asspecified in claim 16 wherein the reaction is continued until thecopolyester resin being produced has an intrinsic viscosity which iswithin the range of about 0.6 dl/g to about 1.0 dl/g.
 20. A process asspecified in claim 18 wherein the reaction is continued until acopolyester resin having an intrinsic viscosity which is within therange of about 0.75 dl/g to about 0.9 dl/g is produced.
 21. Acopolyester resin made by the process specific in claim
 2. 22. Asolution adhesive made by dissolving the copolyester resin made by theprocess specified in claim 2 in an organic solvent.